FIG. 1 is a block diagram of a prior art memory system. The system of FIG. 1 includes three memory modules 10, 12, and 14 that are coupled to a memory controller 16 through a bus 18. Each memory module is fabricated on a circuit board that plugs into a connector 20 on a mother board 22. Each module includes multiple memory devices 24, 26, and 28 that are coupled to the bus 18 to allow the memory controller to access the memory devices.
To increase the memory density of the modules, memory devices can be stacked on top of each other, thereby increasing the memory capacity of each module without increasing the space required on the circuit board. Stacking memory devices, however, increases the capacitive loading of the signals on the bus. For example, from the perspective of the memory controller 16, each data line in the bus 18 has a total capacitance that equals the sum of the capacitance of each portion of the signal line running through sections A, B, and C of the bus, plus the capacitance of the portion of the data line in sections 30, 32, and 34 that couple the memory devices to the bus, plus the sum of the input capacitance of all of the memory devices (which are attached to sections 30, 32, and 34 in parallel). If additional memory devices are stacked on devices 24, 26, and 28, then the capacitance of the additional devices are added to the total capacitance seen by the controller. Therefore, when the memory controller drives a data signal onto the bus, it must overcome the combined capacitance of all of the stacked memory devices. This heavy capacitive loading reduces the maximum operating speed and increases the power consumption by the memory system, especially at higher operating frequencies.